1. Field of the Invention
The present invention relates to a wire electrical discharge machining apparatus and, more particularly, to the improvement of the holding mechanism and the mounting and dismounting operation of a surrounding member which is provided at the tip of a nozzle for jetting a machining liquid so as to efficiently supply the machining liquid to a spark gap.
2. Description of the Prior Art
FIG. 9 is an explanatory view of a conventional wire electrical discharge machining apparatus. A wire electrode 1 is supplied from a supply bobbin 2, and a brake roller 3 is connected to a solenoid brake 3a so as to impart a predetermined stretching force to the wire electrode 1. Idlers 4a, 4b and 4c are provided for changing the direction of travel of the wire electrode 1. A guide 5 is disposed in an upper nozzle 7 for a machining liquid so as to determine the upper position of the wire electrode 1, and a guide 6 is disposed in a lower nozzle 8 for a machining liquid so as to determine the lower position of the wire electrode 1. A pump 9 supplies a machining liquid 10 to the nozzles 7 and 8, and the machining liquid 10 is supplied to the machining portion from the respective jet holes 7a and 8a of the upper and lower nozzles 7 and 8. A machining power source 11 causes electrical discharge between the wire electrode 1 and workpiece 12 by applying a voltage. A roller 13 winds the wire electrode 1.
The operation of this apparatus will now be explained. The wire electrode 1 is supplied from the supply bobbin 2, provided with a stretching force by the brake roller 3 which is connected to the solenoid brake 3a, and is wound around the take-up roller 13 while the direction of travel of the wire electrode 1 is changed by the idlers 4a, 4b and 4c in the course of travel.
When the wire electrode 1 and the workpiece 12 are faced to each other through a small gap and a voltage is applied from the machining power source 11, the dielectric breakdown is caused, and the heat energy of discharge melts the metal of the workpiece 12 and removes it in tiny bits from the surface of the workpiece 12. This action is machining. The wire electrode 1 and the workpiece 12 are relatively moved with an appropriate gap therebetween while repeating discharge by a control device and a driving device (not shown), and as a result, an outline of a desired configuration is produced as if by a scroll saw.
At this time, the machining liquid 10 is jetted from the jet holes 7a and 8a of the upper and lower nozzles 7 and 8, respectively, to the spark gap by the pump 9 so as to wash away the molten machining refuse in the gap, recover the insulation, and cool the wire electrode 1 which is exposed to a high temperature by discharge. The gap between the surface of the workpiece 12 and the upper and lower nozzles 7 and 8 is ordinarily set at about 0.1 to several mm, and the width of the groove which has been machined into a predetermined configuration by electrical discharge machining is at most about 0.5 mm. Therefore, all the machining liquid 10 is not supplied to the spark gap, but it is branched into a machining liquid flow 10b which is supplied to the spark gap and a machining liquid flow 10a which flows on the surface of the workpiece 12, as shown in FIG. 10.
In order to increase the machining speed, the discharge energy is increased or the number of times of discharge is increased. In this case, however, the amount of machining refuse produced is increased and the temperature of the wire electrode 1 is raised, thereby disadvantageously making the machining unstable and inducing the disconnection of the wire electrode 1. It is therefore necessary to increase the flow velocity and the flow rate of the machining liquid 10 jetted from the upper and lower nozzles 7 and 8, thereby enhancing the capacity of eliminating the machining refuse and cooling the wire electrode 1. However, even if the supply voltage of the supply pump 9 is increased, most of the machining liquid 10 flows on the surface of the workpiece 12, as described above. That is, the amount of supply of the machining liquid 10 is scarcely increased in the spark gap to which the machining liquid 10 is to be supplied, and the speedup of the machining speed is therefore disadvantageously impossible.
Methods for solving this problem have been disclosed in Japanese Utility Model Laid-Open No. 140134/1984, and Japanese Patent Laid-Open Nos. 61717/1986 and 152326/1986 and are used. These are methods of providing a surrounding member 14 at the tip of the upper nozzle 7 and/or the lower nozzle 8 substantially coaxially with the wire electrode 1 so as to prevent the machining liquid 10 from being branched into the machining liquid flows 10a and 10b, as shown in FIGS. 11, 12 and 13. According to these methods, the machining liquid flow 10a which is about to flow on the surface of the workpiece 12 is intercepted by the surrounding member 14 and is supplied to the spark gap.
In a conventional wire electrical discharge machining apparatus having the above-described structure, since the surrounding member 14 slides on the surface of the workpiece 12, the surrounding member 14 is worn or is broken due to friction. The surrounding member 14 is therefore required to be easy to replace, and also to have a function of effectively intercepting the machine liquid flow 10a without leakage.
The conventional apparatus, however, cannot fully display these functions.
In the apparatus disclosed in Japanese Utility Model Laid-Open No. 140134/1984 and shown in FIG. 11, the surrounding member 14 is constituted by an O-ring and is inserted into the groove at the tip end surface of the nozzle 7 or 8. Therefore, although the replacement of the surrounding member 14 is easy, when the surrounding member 14 is separated from the workpiece 12 at a preparatory stage (the machining liquid 10 is not jetted) or at the time of finishing the roughly machined outline (the pressure of the machine liquid 10 is low), the surrounding member 14 is inconveniently slipped off the nozzle 7 or 8 under the weight of its own gravity or due to a shock.
In the apparatus disclosed in Japanese Patent Laid-Open No. 61717/1986 and shown in FIG. 12, the surrounding member 14 is composed of a sponge-like member. Therefore, when the pressure of the machining liquid supply pump 9 reaches approximately more than 10 kg/cm.sup.2, deformation of the member by the machining liquid pressure and leakage from a hole are produced. Thus, this apparatus cannot be put to practical use.
In the apparatus disclosed in Japanese Patent Laid-Open No. 152326/1986 and shown in FIG. 13, since the surrounding member 14 is engaged with the recessed portion of the outer peripheral wall of the tip of the upper nozzle 7, the replacement of the surrounding member is not easy and the nozzle 7 as a whole must be replaced.